As an automobile is being fueled with gasoline at a service station, gasoline flowing into the fuel tank displaces gasoline vapor which, unless collected, escapes into the atmosphere. Such vapors not only contribute to atmospheric pollution, but also are unpleasant to the person operating the nozzle, and may adversely affect the person's health over a longer term. As a result, some governmental authorities forbid releasing these vapors into the atmosphere and require collection of any excess vapor for retention and recycling. In the past, various systems have been proposed and used for collecting and returning these vapors to a storage vessel, typically the underground storage tank from which the gasoline is being dispensed. The vapors thus stored are typically then collected for subsequent disposal by the over-the-road tanker when it delivers additional fuel to the storage tank, or are disposed of by other means.
In one such prior art system, the dispensing pump nozzle is sealed to the fuel tank filler neck so that the displaced fuel vapor is directed to the underground storage tank by way of an annular conduit around the nozzle, a coaxial dual conduit hose attached to the nozzle, and appropriate attached plumbing. The design of the nozzle necessary to effect such a seal to the fuel tank filler neck has generally involved the addition of a bellows around the nozzle spout which operates to seal the annular vapor recovery passageway to the filler neck of the tank, as well as various other modifications which make the hand-held nozzle heavy and cumbersome, thereby causing the fueling process to be quite difficult, onerous and unreliable, particularly for the self-serve motorist.
The problems relating to sealable bellows nozzles have been somewhat mitigated by a system which utilizes a vacuum pump to assist in the collection of excess fuel vapor and its transfer to the storage tank. As a result of the use of the vacuum pump, it is unnecessary to seal the vapor recovery passageway to the filler neck of the tank with a bellows, hence reducing the weight of the nozzle and simplifying the fueling process. In this "bellowless" system, the vacuum vapor recovery inlet need only be placed in close proximity to the filler neck of the fuel tank. However, it is very important in this system that the volume of gaseous mixtures drawn in through the vapor recovery vacuum inlet closely approximate the volume of vapor being displaced by the gasoline flowing into the fuel tank. If the volume of vapor being collected is less than that discharged from the tank, it will obviously result in some vapor escaping into the atmosphere. On the other hand, if the volume of vapor collected is greater than the volume discharged from the fuel tank, excess air may be recovered with the vapors, which can create a hazardous vapor/air mixture in the storage tank.
One previous bellowless system controls the appropriate ratio of excess fuel vapor recovered to fuel dispensed by a positive displacement vacuum pump which is driven by a hydraulic motor, which is in turn driven by the flow of gasoline being dispensed into the fuel tank. A major disadvantage of this type system is that a relatively expensive pump unit is required for each dispensing hose or nozzle. In addition, the large number of individual nozzles associated with each typical multi-grade dispensing unit results not only in complex and expensive plumbing, but also occupies substantial space. Thus, the total cost of such a system is a deterrent to its widespread adoption. Also, the hydraulic motor causes an undesirable drop in the pressure (and hence the flow rate) of the gasoline.
A second previous bellowless system measures the rate of flow of gasoline dispensed into the fuel tank and operates an electrically driven vapor pump at a rate having a fixed relationship to the flow of gasoline, modified only by the measured pressure on the intake side of the vapor pump. For example, if empirical data indicate that on average 300 cubic inches of fuel vapor are displaced for every gallon of fuel dispensed, the vapor pump would be controlled to draw 300 cubic inches of vapor for every gallon of fuel dispensed.
A third previous bellowless system measures the temperature of the gasoline in the storage tank, the temperature of the recovered vapors, and the density of the recovered vapors. From these measurements, the system calculates the proper rate at which to drive a vapor recovery pump.
All of these prior art systems suffer from similar disadvantages. They rely on a calculation based on a pre-set formula derived from average empirical data in order to determine how much vapor should be recovered from the fuel tank. The accuracy of the vapor recovery rate is determined only by the accuracy of the formula, and is not verified during operation. The first and second systems do not take temperature of the system into account, which can affect the amount of fuel vapor displaced. None of the prior art systems can self adjust for different grades of fuel or for variations within the same grade. Also, these systems cannot reliably prevent the escape of significant amounts of fuel vapors to the atmosphere since such escape is not detected directly.